03 April 2009 Riken

Smashing Pictures

Controlled collisions of molecules allow RIKEN scientists to visualize dual microscopic chemical reaction pathways

A research team at RIKEN's Advanced Science Institute in Wako has broken the rules of chemical reactions. Instead of using test tube sets, the researchers generated high-velocity beams of atoms and molecules. By imaging the collision process, they were able to visualize the exact reaction pathway taken during the creation of new molecular species.

Three-dimensional dynamic imaging of a chemical reaction.

Figure 1: Three-dimensional dynamic imaging of a chemical reaction. Most excited oxygen atoms (orange spheres) undergo a glancing collision with methane molecules (black/blue spheres), producing a large, bright distribution of CH 3 in the forward direction via insertion (top). Only head-on collisions of atoms and molecules produce discreet rings of CH 3 in the backward direction via abstraction (bottom).

The team, led by Toshinori Suzuki, is a world leader in the field of imaging molecular collisions. In recent work, the researchers tackled how electronically excited oxygen atoms (O*) react with methane (CH 4 ) gas 1 . This reaction is one of the most important primary processes in the stratosphere.

First, Suzuki and colleagues used a laser to generate the O* atoms needed to reproduce the reaction in the laboratory. Then, they crossed accelerated beams of methane and O* gas at right angles in a vacuum chamber, smashing the gases together. The collisions produced a large amount of the methyl radical species CH 3 , which could be ionized and projected onto a phosphor screen, as in a cathode-ray TV.

The projected images showed how CH 3 scatters away from the interaction center at different velocities, and dramatically revealed the co-existence of two chemical reaction pathways (Fig. 1). The CH 3 products either scatter forward, in a large continuous distribution, or backward, as discrete concentric rings.

When the team crossed beams of O* and methane, they found that particles usually undergo a glancing collision, hitting each other's sides. This contact inserts oxygen between a carbon and hydrogen atom, forming a methanol intermediate in its ground electronic state that quickly breaks up into two products: CH 3 , which continues in the same forward direction as the original methane beam, and OH, which moves in the opposite direction.

The torque from the glancing collision produces a broad distribution of forward scattered CH 3 . Because energy is conserved in the collision, the continuous range of CH 3 velocities directly indicates that the other product, OH, is vibrating and rotating considerably.

Backward scattered products are much rarer, and happen when O* directly abstracts a hydrogen to produce OH and CH 3 via the excited electronic state of CH 4 -O*. “For the reaction to occur, the oxygen, hydrogen, and carbon atoms must lie in a straight line, and collide head-on,” explains Yoshihiro Ogi, a postdoctoral researcher in the group. “The discrete rings indicate that the OH product is vibrating, but not rotating, upon formation.”

“This experimental technique will continue to reveal much about gas-phase reactions, especially those related to atmospheric chemistry,” says Ogi.

Source: Riken /...

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